1. Field of the Invention
This invention relates to an image pickup device or an array of image pickup devices in an image sensor that is used for reading image with facsimile or image scanner equipment. In particular, this invention relates to an image pickup device of the type that picks up signals after photoelectric conversion in a plurality of diodes formed of an amorphous or polycrystalline semiconductor.
2. Description of Related Art
Conventional image sensors that are used in reading image with facsimile and image scanner equipment are typically designed in such a way that using a line of image pickup devices of substantially the same length as the document width, image signal is read from one line on the document surface by electrical scanning in the line direction while, at the same time, the document is moved by means of a document feed mechanism (in the slow scan direction) as it is scanned electrically to read information from the entire surface of the document. A typical example of image sensor of this type that has so far been proposed is shown in FIG. 1; a photodiode PD and a blocking diode BD are connected in series, with the polarity of PD being opposite to that of BD, so as to form an image pickup device 100, and a plurality of such image pickup devices are arranged in line one-dimensionally. Such array of image pickup devices are typically formed by the thin-film process using the layer of either amorphous semiconductor (a-Si) or polycrystalline semiconductor.
The process of reading image signal with a single unit of image pickup device 100 in the image sensor shown in FIG. 1 is described below with reference to the timing chart FIG. 2. Suppose first that the photodiode PD is already charged at point b. The photodiode is then illuminated with light reflected from the document surface (not shown), whereupon a photocurrent proportional to the amount of light illumination will flow to the anode of the photodiode PD, causing the potential Vcp at the midpoint between the photodiode PD and the blocking diode BD to be discharged toward the ground so as to store electric charges at the node CP (period b-c, or storage time).
Subsequently, voltage Vh is applied to the anode of the blocking diode BD by application of drive pulses (Vpulse), whereupon the blocking diode BD turns on, causing the junction potential Vcp at the midpoint between the two diodes to become substantially equal to Vh; at the same time, the photodiode PD is charged and reset at the voltage Vh (signal reading and reset time).
When the voltage of drive pulses (Vpulse) drops to Vlow, both the photodiode PD and the blocking diode BD are turned off; if the photodiode PD is illuminated with light while it is in the off-state, the storage time just described above is repeated.
As one can see from the above description, the electric charges stored at the node CP during the storage time are released in the subsequent signal reading time to flow out to an external circuit. In other words, charges of the same amount as the positive charges that flowed out of the cathode of the photodiode PD as photocurrent during storage time will be supplied from an external circuit by the signal reading operation, thereby causing a charging current Iout to flow. An image signal output can be produced by detecting the additionally supplied charges as voltage across a resistor R.
The procedure just described above is repeated for all of-the image pickup devices arranged in line, whereby image signal can be produced for one line across the document surface over time.
A problem with the process of reading image signal with image pickup devices of the composition described above is that an image lag develops in the document's image in the direction of paper feed, thereby reducing the resolution that can be attained. This phenomenon is described below in greater detail.
Suppose first that the photodiode using an amorphous silicon semiconductor layer is illuminated with light of high intensity during the storage time. If a large charging current flows during the signal reading time, the differential resistance of photodiode PD (.DELTA.V/.DELTA.I representing the gradient at point A of the curve shown in FIG. 3) is small and, hence, the time constant expressed by the resistance and the capacity Cp of the photodiode is satisfactorily small.
However, if the illuminating light is of low intensity, only small charges will be stored at the node CP, causing a small charging current to flow during the signal reading time (at reset time); hence, the differential resistance of photodiode PD (.DELTA.V/.DELTA.I representing the gradient at point B of the curve shown in FIG. 3) increases. As a result, the time constant defined above also increases, prolonging the time required for the junction potential Vcp to saturate. This may occasionally cause the photodiode to be reset only insufficiently within a preset signal reading time. In other words, the time constant of the photodiode PD is variable with the amount of incident light and, hence, if it is illuminated with low-intensity light, residual charges will develop and when the next line on the document is read, an afterimage may occur to lower the resolution.